The use of visible, near-infrared (NIR), and low-energy ultraviolet light in clinical practice is growing rapidly. Compounds absorbing or emitting in the visible, NIR, or long wavelength (UV-A, >350 nm) region of the electromagnetic spectrum are potentially useful for diagnostic techniques such as tomographic or planar imaging, endoscopic examination, optoacoustic imaging and sonofluorescene imaging.
Notwithstanding the importance of photodiagnostic applications, a major advantage of biomedical optics lies in its therapeutic potential. Phototherapy has been demonstrated to be a safe and effective procedure for the treatment of various surface lesions, both external and internal. Its efficacy is comparable to radiotherapy, but without the harmful radiotoxicity to critical non-target organs.
The use of fluorescent dyes and covalent dye conjugates for the detection of various species in body fluids is well known in the field of in-vitro immunodiagnostics. Fluorescent dyes have also been used as chemosensors and in fluorescence microscopy. Dyes and dye-immunoconjugates have been widely used in immunohistology, and in immunochemical detection of various small and large biomolecules in body fluids using enzyme-linked immunosorbent assay (ELISA) procedures. More recently, indocyanine green, a highly fluorescent polyene dye, has been used for monitoring cardiac output, assessing hepatic function, and tomographic imaging of tumors.
Specific targeting of fluorescent dyes to a particular site, such as a tumor, has advantages over non-specific localization of these dyes in various tissues. A known method of targeting is by attaching the dye or other effector molecule to an antibody that binds at a target site. Antibodies can tolerate the attachment of a reasonable number of haptens, such as up to about five haptens, on their surfaces while still substantially retaining their binding properties. In contrast, conjugation of haptens to molecules that are similar in size, such as drugs or hormones, most often but not always, obviate the binding properties of the effector molecule to the receptor. This is due to the fact that the large size of antibodies permits the attachment of haptens away from the combining site of the antibodies.
We have demonstrated that indocyanine green dye covalently attached to octreotate retains the somatostatin receptor binding properties, as reported in Achilefu et al., Investigative Radiology, 2000, vol. 35, p. 479, which is expressly incorporated by reference herein in its entirety. This observation, however, is not general and it is not possible at this time to predict a priori the binding properties of small molecule bioconjugates with a high degree of confidence. In contrast, the binding properties of antibody conjugates are generally predictable.
To target a receptor using antibodies, however, anti-receptor antibodies are required. Conventional methods of producing anti-receptor monoclonal antibodies require the isolation of pure receptors. This is often not possible for various reasons, including the lack of stability of many biological receptors. Thus, most receptor targeting with anti-receptor antibodies have been elusive.
The idiotypic network theory of Jerne (Immunological Reviews, 1984, 79, 5-24) proposes that the variable regions of antibodies (i.e. idiotypes) act as immunogens to give rise to a secondary set of antibodies, called anti-idiotypes. An anti-idiotypic antibody is an antibody raised against a first antibody.
The binding site of an antibody is the particular region of the antibody molecule which specifically binds to the recognized epitope. In particular, if antibodies are developed against a ligand that binds to a certain receptor within the body, then a subpopulation of the resulting anti-idiotypic population, which is referred to as ‘internal images’, may contain antibodies that will likewise bind to the same receptor, due to the sharing of a common epitope between the ligand and the internal image. Essentially, the anti-idiotypic antibody mimics the original ligand or drug that is specific to the particular biological receptor.
Application of the principles proposed by Jerne has led to the isolation of a number of internal image antibodies directed at various biological receptors, without ever having to isolate and purify the natural receptor. Examples of these receptors include receptors for thyroid stimulating hormone (TSH), glucocorticoid, and adenosine. Lue et al. (Proceedings of the National Academy of Sciences, 1994, 91, 10690-10694) have used this approach to study the mechanism of an anti-cancer compound, taxol. Thus, the use of internal image antibodies with photoactive molecules for photodiagnostic and phototherapeutic purposes is desirable.